1. Field of Invention
This invention relates generally to membranes which are useful as ultrafilters and microporous membranes useful in separating materials. More particularly, this invention relates to improved integral asymmetric membranes having a skin and a porous support.
2. Description of the Prior Art
Polymeric membranes are well known. These membranes may generally be classified according to their retentivity, i.e., the sizes of particles which they retain, or according to their effective pore size, as either ultrafilter membranes, which have the finest pores, or microporous (or microfilter) membranes which have coarser pores. The dividing line between ultrafilter membranes and microfilter membranes is between approximately 0.025 and 0.050 micrometers in pore size or smallest retained particle.
Membranes may also be classified according to the porosity difference or similarity of their two faces. Thus, membranes may be classified as symmetrical, when the two faces have similar porosity or as asymmetrical when the two faces differ in porosity.
An important characteristic of a membrane is its permeability to water which is measured by the volume of pure water which passes through a unit area of membrane per unit time. Water permeability is customarily expressed in units of cm/min-psi which represents the macroscopic velocity in cm/min at which water flows through the membrane when the driving pressure is one psi.
The flow of water through the membrane is, within wide limits, directly proportional to the applied pressure. In general, the permeability to water decreases as the retentivity of the membrane to solutes increases, because smaller pores offer more resistance to flow. This relationship, however, is not a simple one since the retentivity depends on the single smallest pore encountered by the liquid in passing through the membrane, whereas the resistance to flow depends on the cumulative effect of all the pores through which this liquid must pass. Hence, membranes of similar solute retention having uniform pores throughout their entire thickness have lower permeabilities than those whose retentivity is due to a thin skin having the same pore size combined with a body or substrate of much larger pores. In other words, symmetrical membranes offer more resistance to fluid flow and therefore have slower flow rates compared to asymmetrical membranes of similar retentivity.
In addition to their retention characteristics, membranes may be characterized by their ability to resist plugging or their dirt-holding capacity. Plugging refers to a reduction of the filtration rate during the filtering operation as a function of the amount of liquid passing the membrane. In order to extend the lifetime of a membrane in a given filtration operation, it is customary to prefilter the fluid through a membrane or filter having higher flow rates and smaller retentivities, but still the ability to reduce severe fouling, or blocking, of the final membrane filter.
Structurally, membranes vary greatly and may generally be classified as either reticulated or granular. In the former, there is a three-dimensional open network of interconnecting fibrous strands and open interstitial flow channels. In the granular type structure, however, incompletely coalesced solid particles called granules leave an interconnected network of pores between them. Reticulated membrane structures generally have a higher porosity than granular membrane structures. As used in the present application, the term bulk porosity is used to mean that percentage of the membrane volume which is open to flow of solvent. It is generally equal, or very nearly so, to (1-bulk density).times.100, where bulk density of the membrane is the weight per unit volume of membrane divided by the weight per unit volume of the membrane polymer.
Polymeric membranes are generally made by preparing a solution of the polymer in a suitable solvent, forming the solution into a thin sheet, a hollow tube or hollow fiber, and then precipitating the polymer under controlled conditions. Precipitation may be carried out by solvent evaporation or by contacting the polymer solution with a nonsolvent.
U.S. Pat. No. 3,615,024 discloses a method of forming porous polymeric membranes which are described as being highly asymmetric. The membranes produced according to that method are only slightly asymmetric, however, and have a permeability to water which is only slightly higher than that of symmetrical membranes of the same retentivity.
Membranes may also be classified as composite, supported or integral. Composite membranes comprise a very thin retentive layer attached to a preformed porous support. In a supported membrane, the actual membrane is attached to a strong sheet material of negligible retentivity. Integral type membranes are formed in one and the same operation having layers of the same composition. These layers may have very different properties, depending, in general, on whether the membrane is symmetrical or asymmetric.
The search has continued for improved, highly asymmetric, membranes having improved retention properties and enhanced flow rates as well as for processes for producing these membranes. This invention was made as a result of that search.